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Soil Sampling
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In order
for a crop to reach its full potential, the soil that it is growing in must
contain adequate levels of available nutrients.
If levels of any essential element are inadequate, or if the pH of the
soil is rendering them unavailable, top yields cannot be achieved.
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Soil
testing to assess the fertility level of a soil is basic to a successful soil
fertility program. Soils of
Alabama are typically low in one or more of the essential plant nutrients and
most are naturally acidic. In
order to adjust nutrient levels and pH’s to optimum levels for crop
production, accurate soil samples should be taken.
Soil samples that are taken improperly may not represent the best
average fertility level for a field and may lead to poor nutrient management.
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What is an accurate
sample?
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Here are
some things to consider when sampling soil:
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Depth
of sampling
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Number
of cores
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Size
of sample area
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Timing
of sampling
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Sampling
pattern over the field
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Handling
of Sample
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Depth
of sampling
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Soil
sampling depth is crucial for accurate representation of what is available in
soil for plant uptake. The idea
here is to examine nutrient levels in the volume of soil that will be
supplying the bulk of the nutrients. The
depth that is sampled depends on crop and tillage practices being used.
Placing a mark on the soil probe at the desired depth helps to insure
that all cores are taken at the same depth.
In conventional tillage, samples should be taken to the depth of the
plow layer, usually about 8 inches. Reduced
tillage and no-till fields should be sampled to about 4 inches.
Pastures should also be sampled at 4 inches.
However, deeper samples to the depth of rooting of the grass may be
helpful in determining levels of nutrients at deeper levels.
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Number
of cores
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Usually 15
to 20 cores per sample. However,
this amount may need to be increased in some cases.
For example, when a pasture has been plowed for the first time in
several years, as in peanuts following bahiagrass pasture, more cores should
be pulled to overcome the effects of incomplete mixing of soil, plant material
and manure. Also, if the previous
crop was sidedressed with fertilizer, or had fertilizer banded at planting,
and the field has been plowed, then the number of cores should be increased to
lessen the effect of any bands that may be sampled.
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Size
of sample area
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The smaller
the area the more accurate the results, but the sampled area should be no
smaller than can be treated individually.
In fields that are uniform in appearance, soil type and past treatment,
about 20 acres is an acceptable goal for each sample. Areas that are visibly different should be treated
separately.
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Timing
of sampling
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The primary
point to remember is to sample during the same season every year in
intensively worked fields in order to better watch trends in a fertility
program. It is probably best to
sample during the fall for spring planted crops and in the spring for fall
planted crops.
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Sampling
pattern over the field
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Typically,
a zigzag pattern over the field, taking cores at random, provides a good
representation of a uniform field. If
the field has not been plowed, then care should be taken to avoid areas in
rows where fertilizer was sidedressed or banded.
Other places to avoid in a field include fertilizer and lime dump
sites, and areas where livestock have congregated.
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Handling
of Sample
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Cores
should be placed into a plastic bucket as they are being pulled.
Galvanized buckets may contaminate samples if micronutrients are being
tested. The cores should be
thoroughly mixed in the bucket, and an adequate portion should be placed into
the soil sample bag. Most sample
bags have a fill line marked on them, but if not, about a cup of soil will be
adequate.
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Complete records should be kept, including field names,
cropping history, when sampled and when and how fertilized. This information along with the sample results will help to
make sound fertility decisions.
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